Don't they make some kind of models of planets and stars? That you put together yourself. May be kind of cheesey and childish but I might consider getting something like that for myself. A lot of the stuff seems to be for kids though.
You could also make a t shirt with some relevant slogan and design. Design it yourself or NASA has a space store with some items. You can also buy part of a meteorite from NASA for like $25 I think.